Field of the Invention
The present invention is broadly concerned with detecting smoldering fires in processing equipment with continuous air flow. More particularly, the present invention is concerned with using decentralized carbon monoxide or other trace gas detectors and centralized signal processing to detect smoldering fires in spray dryers, dust collectors, mills, and other processing equipment that operate with continuous air flow.
Description of the Prior Art
In spray dryers, a liquid or slurry of a product is atomized to small droplets and then dried by sudden contact with hot air to powder particles having particular particle sizes. Spray dryers can be found in the food and diary industries, where they are used, for example, in the production of milk powder, cacao, and coffee powder, and also in the pharmaceutical and chemical industries. Slow exothermic reactions of powder deposits in hot regions of drying chambers may evolve over a critical residence period into smoldering fires, which may ultimately result in open flames and/or dust explosions. The likelihood of such events can be reduced by controlling drying temperatures and following thorough cleaning practices.
One way to detect and prevent smoldering fires involves monitoring exhaust air temperature. This solution suffers from several problems and limitations, including that, as a result of high air flow through the drying system, high heat associated with large fires already in progress is required before such systems detect and attempt to extinguish the fires (by, e.g., activating a sprinkler system). Another way involves monitoring carbon monoxide (CO) concentrations in an attempt to detect smoldering fires before open flames or explosions occur. More specifically, the smoldering phenomenon is a slow, flameless combustion that emits CO gas, carbon dioxide (CO2) gas, water (H2O) vapor, and heat. Because CO2 and H2O are more abundantly present in normal air, CO is the most distinctive reaction product. Moreover, some solid organic substances of the smoldering powder are decomposed and evaporated due to the heating, and trace gases other than CO are emitted. Current smolder detection systems are based on CO measurement and use sensitive but expensive infra-red or laser gas analyzers. Because of the high cost of such analyzers, multiple sample points are typically connected to a single centralized sensor which analyzes the samples sequentially. This solution also suffers from several problems and limitations, including long and vulnerable gas sampling lines, delays in detection, and difficulty with filtering stray gases that enter the system from the outside, so that, in practice, the full sensitivity of such sensors is unrealized. For example, fast gas analyzers may have a measurement cycle of approximately fifteen seconds, so if three air streams are sequentially monitored, the smoldering alarm may be delayed by up to forty-five seconds.
A similar risk of smoldering fires arises in other processing equipment involving powdered material and heat. For example, mills break solid materials into smaller pieces by grinding, crushing, or cutting. Similar risks of fires or explosions can arise in mills due to the heat of mechanical friction or impacts, which may be exacerbated by hot air streams, which can cause deposits to begin smoldering.
Dust collectors, which are often used downstream of dryers and mills to separate dust particles from the exhaust air stream, are also exposed to smoldering risk. The risk is created by glowing embers produced by the upstream dryer or mill, transported pneumatically to the dust collector, and which settle into and ignite the collected dust in the hopper of the dust collector.